P-type single crystal zinc-oxide having low resistivity and method for preparation thereof

ABSTRACT

The present invention provides a low-resistivity p-type single-crystal zinc oxide. An n-type dopant and p-type dopant are doped into zinc oxide with higher concentration of the p-type dopant than that of the n-type dopant during forming a single-crystal of the zinc oxide through a thin film forming process. Further, an element of the second group is co-doped to allow oxygen to be stabilized.

TECHNICAL FIELD

The present invention relates to a low-resistivity p-type single-crystalzinc oxide (ZnO) and a method of preparing the same.

BACKGROUND ART

It has been possible to prepare a low-resistivity n-type ZnO by aconventional B, Al, Ga or In doping technique without any difficulty.However, as for p-type ZnO, there have been only reports on ahigh-resistivity p-type ZnO obtained by N (Nitrogen) doping. Forexample, one N-doped p-type ZnO was reported from Kasuga Laboratory ofEngineering Department of Yamanashi University in the 59th Meeting ofthe Japan Society of Applied Physics (Lecture No. 17p-YM-8, JapaneseJournal of Applied Physics, Part 2, vol. 36, No. 11A, p. 1453, 1 Nov.1997). The p-type ZnO thin film prepared by Kasuga Laboratory ofEngineering Department of Yamanashi University is not suitable forpractical use because of still high resistivity of 100 Ω·cm. Further,the p-type ZnO thin film has a remaining problem of experimentalrepeatability in that its conduction type is inversely changed fromp-type to n-type after annealing, and has not been developed to aneffective low-resistivity p-type ZnO.

Li is one element of the first group of the Periodic System and isassumed as an acceptor for fabricating p-type materials. Heretofore,Li-doping has been used only for fabricating high-resistivity ZnO thinfilms, and such doping effects are being studied in a field ofdielectric materials as electrical insulators rather than materials forsemiconductor devices. For example, in the journal of the PhysicalSociety of Japan, vol. 53, No. 4, pp. 282-286, Akira Onodera (GraduateSchool of Science, Hokkaido University) has reported to prepare aLi-doped ZnO having a high resistivity (specific resistance) of 10¹⁰Ω·cm as a memory material by one crystal growth method, so-calledhydrothermal method.

(Problem to be solved by the invention)

Out of ZnO having a p-type conductivity, various high-resistivity p-typeZnO thin films can be fabricated, whereas it has been difficult to growa ZnO single-crystal thin film having a low resistivity with keeping thep-type conductivity due to self-compensation-effects and low solubilityof p-type dopants.

If a p-type ZnO having a low resistivity can be synthesized as a ZnOsingle-crystal thin film, it becomes possible to achieve a p-n junctionbetween ZnO (zinc oxide) semiconducting compounds of the same kind bycombining the synthesized low-resistivity p-type ZnO with thelow-resistivity n-type ZnO (zinc oxide) which has already been put intopractice through the impurity doping process using B (Boron), Al(Aluminum), Ga (Gallium) or In (Indium). This p-n junction, referred toas a homojunction, makes it possible to fabricate various semiconductordevices, such as an implantation type light-emitting diode, laser diodeand thin film solar cell, with high quality and low cost. For example,the above ZnO can be used for fabricating an ultraviolet laser diodenecessary for high-density recording or large-scale informationtransfer.

DISCLOSURE OF INVENTION

(Means for solving the Problem)

In order to solve the above problem, the inventors have developed anovel doping method for incorporating a p-type dopant into ZnO toachieve enhanced stabilization in ZnO.

More specifically, the present invention is directed to alow-resistivity p-type single-crystal ZnO containing a p-type and ann-type. The present invention is also directed to a low-resistivityp-type single-crystal ZnO containing a p-type, an n-type, and the secondgroup element.

In the low-resistivity p-type single-crystal ZnO according to thepresent invention, the n-type dopant may be one or more elementsselected from the group consisting of B, Al, Ga, In, Zn, F, Cl and H.Further, the p-type element may be one or more elements selected fromthe group consisting of the first group elements, the fifth groupelements and C, preferably of Li, Na, N and C.

The concentration ratio of the contained p-type dopant to the containedn-type dopant is preferably set in the range of 1.3:1 to 3:1, mostpreferably in 2:1. The low-resistivity p-type single-crystal ZnOaccording to the present invention has a hole concentration of2×10¹⁸/cm³ or more, more preferably 1×10¹⁹/cm³ or more. Further, thelow-resistivity p-type single-crystal ZnO has an electrical resistivityof 20 Ω·cm or less, more preferably 10 Ω·cm or less, more specificallyless than 1 Ω·cm.

The n-type dopant and p-type dopant are pared up in a ZnO single-crystalby doping the n-type and p-type dopants into the ZnO single crystal. Inthis state, carriers will be scattered not by the p-type dopants with acharge but by p-type dopants with a smaller charge, to be screened byvirtue of the n-type dopant having an opposite charge to that of thep-type dopant. This leads to the realization of the mobile carriers.Thus, the hole mobility of carriers is significantly increased, andthereby a desired low-resistivity p-type single-crystal ZnO can beobtained.

The second group elements of the Periodic System have no influence onthe conduction type, and contribute to stabilization of oxygens in thebasal ZnO semiconducting compound to carry out a function of reducingthe concentration of oxygen vacancy. In the second group elements, Mgand/or Be are particularly desirable to achieve this function.

Further, the present invention is directed to a method of preparing alow-resistivity p-type single-crystal zinc oxide in which an n-typedopant and p-type dopant are doped into zinc oxide with higherconcentration of the p-type dopant than that of the n-type dopant duringforming a single-crystal of the zinc oxide through a thin film formingprocess.

Further, the present invention is directed to a method of preparing alow-resistivity p-type single-crystal zinc oxide in which n-type andp-type dopants and at least one of Mg and Be are doped into zinc oxidewith higher concentration of the p-type dopant than that of the n-typedopant and that of the at least one of Mg and Be during forming asingle-crystal of the zinc oxide through a thin film forming process.

In this method of preparing a low-resistivity p-type single-crystal zincoxide according to the present invention, the thin film forming processmay include the step of supplying an atomic gas from a Zn solid sourceand an active oxygen onto a semiconductor substrate to grow asingle-crystal zinc oxide thin film on the substrate.

In the above methods of preparing a low-resistivity p-typesingle-crystal zinc oxide according to the present invention, an atomicgas vaporized from a Zn solid source by use of MOCVD (Metal OrganicChemical Vapor Deposition) or MBE (Molecular Beam Epitaxy) using anatomic beam and an active oxygen may be flowingly supplied onto anddeposited at a low temperature on a semiconductor substrate to grow asingle-crystal zinc oxide thin film on the semiconductor substrate undersufficient oxygen plasma. During this process, the p-type and n-typedopants and at least one of the second group elements are doped into thezinc oxide.

According to the present invention, the doped p-type and n-type dopantscan suppress the increase of electrostatic energy due to Coulomb'sreaction force between the p-type dopants, and can bring out Coulomb'sattraction force between the n-type and p-type dopants to create anenergy gain. The gain from the above electrostatic interaction allowsmore p-type dopants to be effectively incorporated, so as to achievefurther enhanced stabilization of the ionic charge distributions in ZnO.Thus, the p-type dopant can be stably doped in a high concentration.While the n-type and p-type dopants may be doped separately at differenttimings, it is desired to simultaneously dope or co-dope them.

The higher concentration of the p-type dopant than that of the n-typedopant can be specifically provided by adjusting their amount to beincorporated or the pressure of the atomic gas. The p-type dopant and/orthe n-type dopant and/or the second group element can be transformedinto an atomic form by electronically exciting them with the use ofradiofrequency waves, lasers, X-rays or electron beams. Preferably, thesubstrate has a temperature ranging from 300° C. to 650° C. Thetemperature less than 300° C. causes extremely reduced growth rate ofthe thin film, which is not suitable for practical use. The temperaturehigher than 650° C. causes intensive oxygen escaping, resulting inincreased defects, degraded crystallization and lowered doping effects.The substrate may include a silicon single-crystal substrate, a siliconsingle-crystal substrate having a SiC layer formed therein and asapphire single-crystal substrate. Preferably, the substrate has thesame crystal structure as that of ZnO, and has almost the same latticeconstant as that of ZnO. However, no above currently used substratesmeet this requirement, and thereby there is not any significantdifference in merit and demerit therebetween. Further, the substrate andthin film may interpose therebetween a chromium oxide layer or titaniumoxide layer having an average valve of respective lattice constants ofthe substrate and thin film to reduce unconformity in crystal lattices.

Further, in the present invention, it is preferable that after formingthe low-resistivity p-type single-crystal zinc oxide on the substrate,the formed single-crystal zinc oxide is cooled down to room temperature,and then subjected to a heat treatment at a high temperature ranging,for example, from about 100 to 250° C. under an electric field. Thisallows hydrogen, which in generally behave as a donor, to be removedoutside. The size of energy gap can be freely controlled by combiningthe low-resistivity p-type single-crystal ZnO with the conventionallow-resistivity n-type ZnO (zinc oxide). This provides anoptoelectronics material having a high performance over the range fromvisible light to ultraviolet light and a wide range of application in animplantation type light-emitting diode or laser diode. Further, theapplicable area can be expanded to various photoelectric conversiondevices or low-resistivity semiconductors such as a solar cell.

Further, a magnetism-semiconductor hybrid-function element can befabricated by doping a transition metal, Mn, Fe or Co, which is amagnetic element, into the thin layer of the low-resistivity p-typesingle-crystal ZnO.

(Function)

According to the doping effects in the present invention, adonor-acceptor pair (e.g. Li—F—Li or N—Ga—N) is formed in the crystal,(1) to suppress the increase of electrostatic energy due to Coulomb'sreaction force between the p-type dopants, and increase the solubilityof additional p-type dopants, and (2) to allow the scattering extent ofthe dopants acting to the dynamics of holes, which is 100 angstroms ormore in a single doping, to interact in a shorter range of several tensof angstroms, and thus allows average free mobility of carriers to beincreased.

Further, the second group element, particularly Mg or Be, is doped (3)to form a strong chemical bonding between Mg—O or Be—O in the crystal soas to prevent oxygen escaping. The above three effects make it possibleto dope the p-type dopant stably in a high concentration, and theresulting low-resistivity p-type single-crystal ZnO can be used as anoptoelectronics material over the range from visible light toultraviolet light.

While the single crystal ZnO thin film is particularly subject to oxygenescaping, one or more elements selected from the group consisting of B,Al, Ga, In, Zn, F, Cl and H occupy the resulting oxygen vacancy toprevent the degradation of crystallization due to the vacancy formation,and the p-type dopant, typically one or more elements selected from thegroup consisting of Li, Na, N and C, is stabilized at Zn coordination (Ocoordination in case of N) by ionic bonding.

For example, Li and F are used as a p-type dopant and n-type dopant,respectively, and these dopants are doped, for example, by F: Li=1:2. Asa result, a strong chemical bonding is created between the adjacent Fand Li to form a complex of Li—F—Li in the ZnO crystal thin film. If Liis singly doped, the energy in lattice system is increased, and therebyan oxygen vacancy is induced. The vacancy acts as a donor and causes thedegradation of crystallization. Thus, the Li moves between lattices, andthereby the roll of the Li is inversely changed from acceptor to donor.This blocks the creation of a low-resistivity p-type single-crystal ZnOthin film.

On the other hand, in F and Li co-doped crystal, the doped Li isstabilized after the complex is formed and thereby the stabilized Limoves to shallow level. This allows more carriers to be created at lowertemperature (at the temperature closer to room temperature) to provide adesired low-resistivity p-type single-crystal ZnO thin film.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram showing the schematic inner structure ofa vacuum chamber for forming a low-resistivity p-type single-crystal ZnOthin film on a substrate by use of the MBE method.

FIG. 2 is a schematic diagram showing the configuration of p-typedopants and n-type dopants, which is determined by use of thefirst-principles band structure alculation method.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to an example, a method for forming a p-typesingle-crystal ZnO thin film on a substrate by use of the MBE methodwill now be described in detail.

As shown in FIG. 1, a sapphire substrate 2 was placed in a vacuumchamber 1 having a maintained internal pressure of 10⁻⁸ Torr, and anatomic Zn gas and an atomic O gas were supplied onto the substrate 2 tofablicate a ZnO thin film on the substrate 2. The atomic Zn gas wasprepared by heating a Zn solid source having a purity of 99.99999% witha heater to bring it into an atomic form. The atomic O gas was preparedby activating oxygen having a purity of 99.99999% with an RF radicalcell. Each of Li serving as a p-type acceptor and F serving as an n-typedonor was prepared by radiating the microwave level of electromagneticwaves to a corresponding molecular gas or by bringing a correspondingelemental cell into an atomic form under a high temperature. FIG. 1shows an RF (radio frequencies) coil 3, a heater 4, and an elementalcell (Li source) 5, which are used in this method. During forming afilm, F as an n-type dopant and Li as a p-type dopant weresimultaneously supplied onto the substrate 2 at a partial pressures of10-7 Torr and a partial pressure of 5×10⁻⁷ Torr, respectively, to inducethe crystal growth for forming a p-type single-crystal ZnO thin film 6at each temperature of 350° C., 400° C., 450° C. and 600° C. For thep-type single-crystal ZnO thin film obtained at each of the abovecrystal growth temperatures, hole concentration, resistivity and holemobility were determined according to a hole measurement and afour-probe method. Table 1 shows this measurement result with comparingto the case where only Li as a p-type dopant was ingly doped withoutco-doping F as an n-type dopant.

Table 1 also shows the case (2) where Mg and Li were co-doped and thecase (4) where Li, F and Mg were co-doped. In these cases, the Mg wasprepared by radiating the microwave level of electromagnetic waves to acorresponding molecular gas or by bringing a corresponding elementalcell into an atomic form under a high temperature. In case of co-dopingthree of Li, F and Mg, an additional component is only one elementalcell.

As can be seen in the hole concentration (number/cm³) shown in Table 1,higher crystal growth temperature provides higher hole concentration. Ascompared to the result of doping Li singly (1), each of the result ofco-doping Li and Mg (2), the result of co-doping Li and F (3), and theresult of co-doping Li, F and Mg (4) exhibits a hole concentrationhaving larger digits by three or more.

TABLE 1 Hole Concentration Resistivity Substrate (number/cm³) HoleMobility (Ω · cm) Temperature (1) (2) (3) (4) (cm²/V · g) (1) (° C.)10¹⁵ 10¹⁸ 10¹⁸ 10¹⁸ (1) (2) (3) (4) 10⁸ (2) (3) (4) 350 1 2 10 8 — 80 7575 90 20 8 10 400 3 3 15 10 — 85 80 78 60 9 2 3 450 6 8 50 20 — 90 95 8810 0.9 0.3 0.4 600 8 10 90 60 — 93 150 105 2 0.25 0.01 0.08 (1) theresult of doping Li singly. (2) the result of co-doping Li and Mg. (3)the result of co-doping Li and F. (4) the result of co-doping Li, F andMg.

Further, as compared to the result of doping Li singly (1), it is provedthat each of the results (2) to (4) exhibits a hole mobility (cm2/V·g)having larger digits by two or more. In case of co-doping, theresistivity (Ω·cm) in inverse proportion to the product of the holeconcentration and hole mobility has reduced digits by five or more ascompared to the case of singly doping, and goes down to less than 10Ω·cm when the substrate temperature is 400° C. or more.

Further, in the sample of co-doping three elements of Li, F and Mg, evenat a low crystal growth temperature of 350° C., a p-type single-crystalZnO thin film could be obtained with a high hole concentration of 8×10¹⁸(number/cm³). Thus, a p-type single-crystal ZnO thin film having a lowresistivity of 10 Ω·cm could be obtained.

FIG. 2 shows the configuration of two acceptors and one donor in a ZnOcrystal, which is determined by use of the first-principles bandstructure calculation method. As can be seen in FIG. 2, it has beenverified that the crystallographic configuration of Li was stabilized byadding Li as an acceptor and F as a donor in the ZnO crystal, andthereby Li could be doped stably in a high concentration. Mg of thesecond group element is located substantially independently of Li and Fto stabilize oxygen.

INDUSTRIAL APPLICABILITY

As described above, ZnO of the present invention is a novellow-resistivity p-type single-crystal ZnO which has not been achieved,and this single crystal ZnO has a innovative broader range ofapplications. Further, the method of the present invention makes itpossible to obtain the low-resistivity p-type single-crystal ZnOreadily.

1. A low-resistivity p-type single-crystal zinc oxide containing ap-type dopant and an n-type dopant, wherein said p-type dopant is one ormore elements selected from the group consisting of Li and Na.
 2. Alow-resistivity p-type single-crystal zinc oxide as defined in claim 1,wherein said n-type dopant is one or more elements selected from thegroup consisting of B, Al, Ga, In, Zn, F, Cl and H.
 3. A low-resistivityp-type single-crystal zinc oxide containing a p-type dopant, an n-typedopant, and a second group element.
 4. A low-resistivity p-typesingle-crystal zinc oxide as defined in claim 3, wherein said secondgroup element is Mg and/or Be.
 5. A method of preparing alow-resistivity p-type single-crystal zinc oxide in which an n-typedopant and p-type dopant are doped into zinc oxide with higherconcentration of said p-type dopant than that of said n-type dopantduring forming a single-crystal of the zinc oxide through a thin filmforming process, wherein said p-type dopant is one or more elementsselected from the group consisting of Li and Na.
 6. A method ofpreparing a low-resistivity p-type single-crystal zinc oxide in whichn-type and p-type dopants and at least one of Mg and Be are doped intozinc oxide with higher concentration of said p-type dopant than that ofsaid n-type dopant and that of said at least one of Mg and Be duringforming a single-crystal of the zinc oxide through a thin film formingprocess.
 7. A method of preparing a low-resistivity p-typesingle-crystal zinc oxide, as defined in claim 5 or 6, which furtherinclude the step of supplying an atomic gas from a Zn solid source andan active oxygen onto a semiconductor substrate to grow a single-crystalzinc oxide thin film on said substrate.
 8. A method of preparing alow-resistivity p-type single-crystal zinc oxide, as defined in claim 5or 6, which further include the step of cooling said formed thelow-resistivity p-type single-crystal zinc oxide on said substrate, andthen subjecting said cooled single-crystal zinc oxide to a heattreatment at a high temperature under an electric field.